The capacity and speed of communications systems may be increased by transmitting information in optical form over networks composed of optically transmissive nodes, fibers, and the like. High capacity optical communications systems require that many optical signals be frequency division multiplexed in the components of an optical network. Receivers must be configured such that a desired optical frequency or band of frequencies may be retrieved from a group of many optical frequencies. This necessitates the provision of suitable optical filters such as a tunable channel dropping filter for dividing out one or more component frequencies from a multiplexed signal composed of a large number of frequencies. Until now, there has been no convenient approach to dividing out one or more optical frequencies with an extremely rapid response time.